ADDITIVES FOR PRIMARY ELECTROCHEMICAL CELLS HAVING MANGANESE DIOXIDE CATHODES

Abstract: The invention relates to primary alkaline cells containing a manganese dioxide cathode active material. Anatase titanium dioxide is added to the cathode. The result is an increase in the useful service life. Typically the anatase titanium dioxide is used in an amount between 0.1 and 5 percent by weight of the cathode active material.

Description: ADDITIVES FOR PRIMARY ELECTROCHEMICAL CELLS HAVING MANGANESE DIOXIDE CATHODES The invention generally relates to primary alkaline electrochemical cells with manganese dioxide cathode active material, and specifically to the addition of anatase titanium dioxide to the cathode material. Conventional Primaryalkaline cells typically contain zinc anode active material, alkaline electrolyte, (such as potassium hydroxide) manganese dioxide cathode active material, and an electrolyte permeable separator film, typically of cellulose. The anode active material comprises zinc particles admixed with electrolyte and one or more gelling agents, such as carboxymethylcellulose or an acrylic acid copolymer. An anode current collector (typically a conductive metal nail) is inserted into the anode active material . The cathode material may include small amounts of carbon or graphite to increase conductivity. Conventional alkaline cells are encased in a steel container to retain the cell components and reduce the chance of leakage. Since commercial cell sizes are fixed it has been desirable to attempt to increase the capacity, i.e., the useful service life of the cell, by increasing the surface area of the electrode active material and by packing greater amounts of the active material into the cell. This approach has practical limitations, since if the active material is packed too densely into the cell this can reduce the rate of electrochemical reaction during discharge, in turn reducing service life. Other deleterious effects such as polarization can occur, particularly at high current drain rates. Polarization limits the mobility of ions within the electrode active material and within the electrolyte, which in turn reduces service life. Thus, it is desirable to provide a way of reliably increasing the useful service life of conventional primary alkaline cells, especially cells having no added mercury, without noticeably increasing polarization effects or otherwise adversely affecting cell performance. It has been discovered that the addition of small amounts of Ti0 2 having an anatase crystal structure to the cathode active material of primary zinc/Mn0 2 alkaline cells increases the discharge capacity of such cells at high and medium drain rates. It has been known to add Ti0 2 to cathodes in rechargeable cells for the purpose of improving rechargeability. See for example Japanese Kokai patent SHO 64-6384; German patent No. DE 3337568; and U.S. patent 5,011,752. However, neither reference teaches that the capacity of a primary cell can be increased by addition of Ti0 2 . It is also known to add Ti0 2 to a cell separator as discussed in U.S. patent no. 5,026,617. The following figure depicts the improved performance of a primary cell incorporating the additive of the invention. Fig. 1 is a graph which compares the performance of a conventionally made Zn/Mn0 2 primary alkaline cell with the performance of a similar cell which is made in accordance with the invention described and claimed herein. The amount of anatase titanium dioxide which is added in accordance with the invention is desirably between about 0.1 and 5 percent by weight of the total cathode material. For C and D size primary zinc/alkaline cells the anatase titanium dioxide is preferably between about 1 and 5 wt% of the total cathode active material. When anatase Ti0 2 is added to the cathode of a "C" or "D" size cell, a 10 to 15 % improvement in service life is realized at high drain rates (discharge at 2 to 4 ohms down to an 0.8 volt cut-off voltage) and an 8 to 13% improvement in service life at medium drain rates (discharge at 4 to 7 ohms down to a 0.9 volt cutoff voltage) . When anatase Ti0 2 is added to the cathode active material in a AA size cell, a 5 % improvement in useful service life is typically obtained at high drain rates (3.9 ohm load) and a 4% improvement is obtained at medium drain rates (10 ohm load) . The above stated improvements in service life are particularly applicable to mercury free zinc/Mn0 2 alkaline cells. As used herein the terms "mercury free" and "cells with no mercury added" are intended to mean that they contain the small residual amount of mercury which is present in commercially available "pure" zinc, as well as any trace amounts of mercury present in the other cell components. The total mercury content in such cells is less than.50 parts mercury per million parts total cell weight, typically less than 10 parts mercury per million parts total cell weight. The improvement in service life is not obtainable with the rutile form of titanium dioxide. It is not known with certainty why the improvement in useful service life occurs. However, it is believed that the addition of small amounts of anatase Ti0 2 to the cathode active material increases the mobility of ionic flow during discharge. This can decrease polarization effects and result in increased service life. The increase in service life is particularly surprising, since such increase is obtained by adding Ti0 2 to the cell at the expense of removing an equal amount of active Mn0 2 material. The present invention is applicable to conventional zinc/Mn0 2 primary alkaline cells. While it is preferred that the zinc anode has no "added mercury, " the improvement of the present invention is also applicable to zinc/Mn0 2 primary alkaline cells containing small amounts of mercury. The invention resides in the addition of anatase titanium dioxide to the cathode material. In other respects the cells are conventional. These cells are typically configured as in U.S. patent 4,740,435 wherein the anode active material forms the central core of the cell and the cathode active material is located around the anode material with the separator therebetween. The cathode material contacts the inside surface of the cell casing which functions as cathode current collector and is typically of stainless steel. The cathode is prepared at room temperature by admixing anatase Ti0 2 , electrolytic Mn0 2 , and graphite. The ingredients are mixed for a short period; e.g., less than one half hour, whereupon 7 Normal KOH is added over a period of several minutes to produce a mixed wet powder. The mix is then formed into annular pellets which are sized in height so that about four can be packed into a steel cell casing to form the cathode. The cavity of the annular cathode structure is lined with an appropriate separator and the anode material is then placed therein. The following examples illustrate the invention and advantaged derived therefrom. (All compositions are by weight unless otherwise specified.) Comparative Example A A primary zinc/manganese dioxide, alkaline, size D cell is prepared with conventional cathode and anode active material, electrolyte and separator membrane. The anode material is in the form of a gelled mixture of mercury free zinc alloy powder. The mixture contains aqueous KOH solution, gelling agent, (acrylic acid copolymer-CARBOPOL C934 from B.F. Goodrich) and surfactants (organic phosphate ester surfactant-GFAC RA600 from Rhone Poulenc) . The electrolyte is an aqueous solution of KOH containing about 40 wt% KOH and 2wt% ZnO, hereinafter referred to as "aqueous KOH solution" . The separator membrane is a conventional electrolute permeable membrane of polyvinyl alcohol/rayon material. The cathode is electrolytic manganese dioxide (84 wt%) , graphite (9.5 wt%) , and a 7 Normal "aqueous KOH solution" (6.5 wt%) . The cell is discharged at a constant high current drain rate of 410 milliamps which is equivalent to about a 2.2 ohm load. The voltage versus time discharge profile for this cell is presented in Fig. 1. The useful service life of the cell as measured by the time needed for the cell voltage to drop to 0.8 volts from its initial voltage of 1.5 volts, is 20 hours. Specific formulations of representative zinc slurries are disclosed in European Patent Publication 0474382A1. Example 1 An experimental zinc/Mn0 2 size D alkaline cell identical to that referenced in example 1 is prepared, except that in making the experimental cell an amount of anatase Ti0 2 is added so that the total cathode material comprised 4.2 percent by weight anatase Ti0 2 . The amount of Mn0 2 in the cathode is reduced by an equal amount so that the total cathode weight in the experimental cell is the same as in the standard cell Comparative of Example A. The cell is discharged at the same constant high drain rate of 410 milliamps. The voltage vs. time discharge profile for this cell is plotted in Fig. 1 next to the profile obtained using the standard cell described in Comparative Example A. It will be seen that the useful service life of the experimental cell (determined at a cut off voltage of 0.8 volts) is 23 hours, which is about 15% longer than the useful service life of the standard cell . The experimental cell running voltage is also about 60 millivolts higher than the standard cell throughout discharge. Example 2 Comparison performance tests are made with different amounts of anatase Ti0 2 additive and at different current drain rates. In a first group of experimental tests the performance of D size zinc/Mn0 2 alkaline cells are compared for sets of cells at three different drain rates (high, medium and low) and at two different levels of amount of added anatase Ti0 2 and discharged at the same drain rate. The composition of the standard cell is as set forth in Comparative Example A. The composition of each experimental cell tested in the I current example is the same as the standard cell, except that various amounts of anatase Ti0 2 have been added to the cathode to yield the specified percent by weight Ti0 2 in the cathode material as reported in Table 1. Accordingly, the amount of Mn0 2 in the cell is reduced by an equal amount of Ti0 2 added so that the total cathode weight in each case remains the same. The performance tests are carried out at both continuous and intermittent discharge regimens. In the latter case the cells are discharged for 1 hour per day every day until the useful service life cut-off voltage has been reached. As each cell is discharged the voltage eventually falls to a point where the cell is no longer useful for the intended service. The cut-off voltages used in determining useful service life are 0.8 volts for high drain rate; and 0.9 volts for each of the medium and low drain rates. The drain rates, amount of anatase Ti0 2 additive and voltage cut-off used to determine the useful service life for each set of cells is summarized in Table 1. The performance results are reported as a percent gain (+) or loss (-) in useful service hours for each set of cells as compared to that obtained with the standard cell. For example, a 10% gain for a set of cells at a given drain rate and amount of anatase Ti0 2 additive, indicates that 10% more service hours are attained for those cells as compared to the standard cell containing no added anatase Ti0 2 . The comparisons for each set of cells is based on an average of results from 10 to 20 identical cells. TABLE 1 Group Test % Ti0 2 Increase In Service Life In Cathode Continuous Intermittent High Rate 2. +13 +10 (2.2 Ohms) 5, +15 +7 II Medium Rate 2 +8 +11 (3.9 Ohms) 5 +8 +7 III Low Rate 2 -2 5 -15 The results presented in Table 1 indicate that very good performance gains in service life are obtained at high (2.2 ohm load) and medium (3.9 ohm load) drain rates. The average gains at high and medium drain rates of the cells tested are about 14 and 8 percent, respectively, at continuous discharge and about 8 and 9 percent, respectively, at intermittent discharge. These gains are obtained at levels of anatase Ti0 2 between about 2 and 5 wt% of the total cathode active material. Service life at low drain rates is reduced. However, the reduction is very small (about 2%) when the amount of anatase Ti0 2 is about 2 wt% of the cathode material. Example 3 Another group of comparison performance tests are made similar to those of Example 2 except that C size zinc/Mn0 2 alkaline cells are used. The experimental "C" cells are made by reducing the amount of Mn0 2 from a standard "C" cell (composition similar to that employed in Comparative Example A) by various amounts and replacing the reduced amount with an equal amount of anatase Ti0 2 . As in Example 2 the performance of sets of experimental cells at different drain rates (high, medium and low) are determined at different levels of anatase Ti0 2 added to the cathode material. The performance results in each set of cells is reported in Table 2 as a percent gain (+) or loss (-) in useful service hours as compared to a standard cell containing no added anatase Ti0 2 and discharged at the same drain rate. As in Example 2 the tests are conducted at both continuous and intermittent discharge regimens (defined above) . The discharge cutoff voltage used to determine the useful service life for each set of cells at high, medium, and low drain rates is the same as given in Example 3. TABLE 2 Group Test % Ti0 2 % Increase In Service Life In Cathode Continuous Intermittent I High Rate 1.25 +11 +ι: (3.9 Ohms) 2.5 +12 +1! 5.0 +7 +7 II Medium Rate 1.25 +9 +12 (6.8 Ohms) 2.5 +10 +12 5.0 +5 +13 III Low Rate 1.25 -4 2.5 -4 5.0 -5 The results presented in Table 2 indicate that very good improvement in service life is attained at high (3.9 ohm load) and medium (6.8 ohm load) drain rates. However, the improvement is greatest at both high and medium drain rates at the lower amounts of anatase Ti0 2 between about 1.25 and 2.5 wt% of the total cathode material. The gains at high and medium drain rates average about 10 and 8 percent, respectively, at continuous discharge, and about 11 and 12 percent, respectively, at intermittent discharge. These average gains are obtained at levels of anatase Ti0 2 between about 1 and 5 wt% of the total cathode material . The average gains for added amounts of anatase Ti0 2 between about 1.25 and 2.5 wt% of the cathode are 12 and 10 percent for high and medium continuous discharge, respectively, and between about 14 and 12 percent for high and medium intermittent discharge, respectively. Service life at low drain rates is reduced. However, the reduction is small (about 4%) when the amount of anatase Ti0 2 is about 1 to 2 wt% of the cathode material . Based on the results of the performance tests presented in the tables, the optimum amount of anatase Ti0 2 to be added to the cathode material in C and D size cells is about 2 wt% of the total cathode material. Similar tests are performed on AA size zinc/Mn0 2 alkaline cells. The best performance improvements attained are at low amounts of added anatase Ti0 2 of less than about 1 wt% of the cathode material. Specifically, with anatase Ti0 2 comprising about 0.6 wt% of the cathode material, a 5 percent improvement in useful service life is obtained on average at high drain rates (3.9 ohm load) and a 4 percent improvement is obtained at medium drain rates (10 ohm load) , both at continuous discharge. Improvements in service life can also be obtained by adding anatase Ti0 2 to the cathode in AAA size cells. The increase in service life reported in Example 1 and Tables 1 and 2 are conservative. Since there is less Mn0 2 in the experimental cells than in the corresponding size standard cell, the percent increase in service life on a per gram Mn0 2 basis would be higher than on the per cell basis reported in Example 2 and the tables. Although the present invention was described with respect to specific embodiments, it should be recognized that variations are possible without departing from the concept of the invention. Thus, the invention is not intended to be limited to the specific embodiments, but rather its scope is reflected by the claims and equivalents thereof.

Claims: What Is Claimed Is: 1. A primary electrochemical cell comprising an anode active material, an aqueous alkaline electrolyte solution, a separator and a cathode comprising manganese dioxide and anatase titanium dioxide. 2. The cell of claim 1 wherein the anode active material comprises zinc. 3. The cell of claim 1 wherein the aqueous electrolyte solution comprises potassium hydroxide. 4. The cell of claim 1 wherein said anatase titanium dioxide is present in an amount between 0.1 and 5 percent by weight of said cathode. 5. The cell of claim 1 wherein the total mercury content in the cell is less than 50 parts per million parts of total cell weight . 6. The cell of claim 1 wherein the total mercury content in the cell is less than 10 parts per million parts of total cell weight. 7. A primary electrochemical cell comprising an anode comprising an active material which comprises zinc, an aqueous alkaline electrolyte solution which comprises KOH, a separator, and a cathode material which comprises manganese dioxide and between about 0.1 and 5 percent by weight of anatase titanium dioxide, and said cell having less than 50 parts mercury per million parts of total cell weight. 10

Inventors: MARKFORT SIMON P, MIECZKOWSKA JOLA E, MARKFORT, SIMON, P., MIECZKOWSKA, JOLA, E.
Applicants: DURACELL INC, DURACELL INC.
Application year: 1994
Main classification: H01M4
IPC codes: H01M4, H01M6